1. Field of the Invention
This invention relates to a semiconductor electronic device, and more particularly, to plastic cases for surface-mount integrated circuits of the thin type.
2. Discussion of the Related Art
As is known, a plastic case, or package, of the surface-mount thin type comprises a supporting plate for a chip of a semiconductor material, most commonly silicon, a plurality of rigid metal conductors connected electrically to selected areas of the chip by small-gauge wire leads, and a body of plastic, usually an epoxy resin, which encapsulates everything but the ends of the metal conductors that form the device rheophores. The rheophores are then bent with their ends lying in a common plane, which plane is substantially the plane of one of the major surfaces of the plastic body.
A recent trend in plastics cases of this type has been toward decreasing thickness, changing from 3-4 mm for the packages of some years ago to 1.4 or 1 mm and even less for current packages.
The supporting plate and metal conductors are formed, usually by a blanking process, from a single strip of sheet metal, as can be seen in FIG. 1. This FIG. shows a sheet metal piece 1 from which supporting plate 2 and two sets of metal conductors 3 and 4 have been formed, each adjacent to one side of the plate. The structure denoted by 1 is called a leadframe, and in general, includes a number of plate/conductor elements.
The metal plate 2 is rectangular in shape and its dimen sions are approximately the same as those of the chip received thereon. In addition, this plate is slightly sunken with respect to the plane of the metal conductors 3 and 4, such that the chip can be located centrally in the plastic case, thereby minimizing the package warping on cooling. Warpage is a frequent occurrence with ultra-thin packages, since the different thermal expansion coefficients of the resin, silicon and metal materials may cause the structure to distort out of plane, unless it is made symmetrical. The chip is secured on the plate by means of a special adhesive, and the metal conductors on the leadframe are connected to selected areas on the chip by thin wire leads.
FIG. 2 shows a mold as used for forming the plastics case. This mold is a split type with an upper mold half 10 and a lower mold half 11, each having a corresponding hollow. The two mold halves are oriented with their hollows facing each other, thereby defining a mold cavity into which the resin will be injected. The portion of the leadframe that is to be encapsulated within the resin is clamped between the mold halves. A synthetic resin 20, e.g. an epoxy resin, is injected into the mold 12 at an elevated temperature through a gate 12.
This process is best illustrated by FIGS. 3, 4, 5 and 6, which FIG.s also highlight some more problems besetting this molding process. As shown particularly in FIG. 4, the resin flows from the gate 12, parts along two paths, one to the upper part of the mold and the other to the lower part. The cross-section of the upper part is larger than that of the lower part; consequently, the resistance met by the resin flow in this part will be less, and the resin flow will be faster and reach the right end of the mold in less time. After reaching the mold end, the resin begins to flow toward the lower part of the mold, as shown in FIG. 5, where there still is space unoccupied, and air becomes trapped 21 at a location from which it cannot be removed.
This occurs particularly in the center region of the package, as shown in FIG. 7, because the velocity of the resin flow admitted centrally through the gate 12 is high along the two sides of the mold and low in the center region. Thus, faults may develop in the package due to trapped air that will unavoidably make a reject of it.
A third problem with packages of this type is related to assembling the devices on the printed circuit boards. For surface mounting, the devices are welded to the printed circuit tracks using a process at a relatively high temperature, typically in the 200 to 250 degrees centigrade, and of relatively long duration (a few tens of seconds). It has been found that this process originates phenomena within the plastics body which affect the physical characteristics and mechanical stability of the package. In particular, with a small ratio of the plastics body thickness to the surface area of the supporting metal plate, tied to the chip dimensions, these effects may result in case rupture.
This phenomenon is known to those skilled in the art as the "pop-corn effect." One theory advanced as to the popcorn effect follows. Water is absorbed by the resin from the environment and is present in the adhesive used to join the chip to the plate. The water migrates toward the outer and inner surfaces of the body. If the adhesion of the plastics body to the parts encapsulated therein is less than perfect or fails because of the different expansions of the materials during the thermal cycle, a void may be formed between the plastic and the chip and between the plastic and the underside of the supporting plate, wherein water can settle. Due to the high temperatures used in welding, the water is vaporized and exerts pressure against the walls of the void. The vapor pressure within the body distorts the package walls, and may ultimately rupture them.
To obviate the destructive effect just described, various measures have been suggested, including removing the absorbed water by baking the parts prior to the welding process, protecting the parts during storage so as to prevent water absorption, and improving the adhesion of the resin to the device components within the body by chemical or mechanical treatment of the supporting plate surface.
All of the measures listed above show more or less serious drawbacks and involve additional processing of considerable cost and difficulty. In view of the current manufacturing trend toward integrated circuits of growing complexity, and accordingly toward the use of chips of increasing size, albeit without increasing the package size, it will be appreciated that the above-described phenomena represent a serious problem in that they restrict the possibility of using certain standard packages.
The underlying technical problem of this invention is to provide a supporting plate having such construction and performance features as to enable a semiconductor electronic device to be mounted within a case of the type outlined above, and which is particularly resistant to the degenerative effects of the welding process and involves no costly alterations of the fabrication process, thereby overcoming the aforementioned limitations of the prior art.